Phosphors are used in fluorescent lamps, vacuum fluorescent displays (VFD), field emission displays (FED), plasma display panels (PDP), cathode ray tubes (CRT), and white light emitting devices. Light emission from a phosphor requires the supply of excitation energy to the phosphor. A phosphor is excited by an excitation source having high energy, such as vacuum ultraviolet light, ultraviolet light, visible light, or an electron beam to emit ultraviolet light, visible light, or infrared light. However, the brightness of light emitted from the phosphor exposed to such an excitation source decreases over time.
Therefore, sialon phosphors whose light brightness is less decreased over time have been supposed an alternative to existing phosphors such as silicate phosphors, phosphate phosphors, aluminate phosphors, borate phosphors, sulfide phosphors, and oxysulfide phosphors.
Sialon phosphors are produced by a hot press process, wherein silicon nitride (Si3N4), aluminum nitride (AlN), calcium carbonate (CaCO3), and europium oxide (Eu2O3) are mixed at a predetermined molar ratio, and the mixture is fired for 1 hour at a temperature of 1700° C. in a nitrogen atmosphere under atmospheric pressure (0.1 Mpa) (for example, see Patent Document 1). The Eu ion-activated α-sialon obtained through the process is then excited by a blue light having a wavelength of 450 nm to 500 nm to become a phosphor emitting yellow light having a wavelength of 550 nm to 600 nm.
In addition to the phosphor emitting yellow light, phosphors emitting an orange or red light have been demanded for some applications such as white light emitting devices and plasma displays which include ultraviolet light emitting diodes as the excitation source. For white light emitting devices which include a blue light emitting diode as the excitation source, a phosphor which emits an orange or red light has been demanded for the purpose of improving color rendering.
As a phosphor emitting red light, an inorganic substance (Ba2−aEuaSi5N8:a=0.14 to 1.16) composed of Ba2Si5N8 crystals activated with Eu has been reported (see Nonpatent Document 1). A phosphor based on a ternary nitride composed of an alkali metal and silicon MbSicNd which may have various compositions (M=Ca, Sr, Ba, or Zn; b, c, and d take various values) has also been reported (see Nonpatent Document 2). In addition, MeSifNg:Eu (M=Ca, Sr, Ba, or Zn; g=2/3e+4/3f) has also been reported (see Patent Document 2).
Other examples of known sialon, nitride, or oxynitride phosphors include phosphors composed of MSi3N5, M2Si4N7, M4Si6N11, M9Si11N23, M16Si15O6N32, M13Si18Al12O18N36, or MSi5AlON10 (M is Ba, Ca, Sr, or a rare-earth element) as the host crystal activated with Eu or Ce, and some of them have been reported as phosphors emitting red light (Patent Document 3). LED lighting units including these phosphors are also known. In addition, phosphors composed of Sr2Si5N8 or SrSi7N10 crystals activated with Ce have been reported (Patent Document 4).
Patent Document 5 describes a LhMiN(2/3h+4/3i):Z phosphor (L represents a divalent element such as Ca, Sr, or Ba, M represents a tetravalent element such as Si or Ge, and Z represents an activator such as Eu; h=2 and i=5, or h=1 and i=7), the phosphor exhibiting reduced afterglow when mixed with a small amount of Al. In addition, a light emitting device composed of the phosphor and a blue LED is known, the device emitting a reddish warm white light. Patent Document 6 describes a LjMkN(2/3j+4/3k):Z phosphor composed of various L, M, and Z elements. Patent Document 7 describes a wide range of combinations of L-M-N:Eu and Z, but does not mention improvements in the luminescence properties through the use of a specific composition or crystal phase as the matrix.
The phosphors typified by those described in Patent Documents 2 to 7 are composed of a nitride of divalent and tetravalent elements as the host crystal. These phosphors emit red light upon excitation with a visible blue light, but the brightness is insufficient. In addition, some compositions are chemically unstable, and present problems in terms of durability.
As existing lighting systems, white light emitting devices composed of a blue light emitting diode and a blue-light-absorbing yellow-light-emitting phosphor are described in Patent Documents 8, 9, and 10. These light emitting diodes most commonly use a yttrium-aluminum-garnet phosphor activated with cerium, which is expressed by a general formula (Y, Gd)3(Al, Ga)5O12:Ce3+.
However, the white light emitting device composed of a blue light emitting diode and a yttrium-aluminum-garnet phosphor emits a bluish white light lacking in a red component, so it exhibits unbalanced color rendering.
Patent Documents 11 and 5 describe white light emitting devices, in which two kinds of phosphors have been mixed and dispersed thereby covering a deficiency of the red component of the yttrium-aluminum-garnet phosphor with the other red phosphor. However, the light emitting devices still have problems to be solved in terms of color rendering. The red phosphor described in Patent document 11 includes cadmium which may cause environmental pollution. The red light emitting phosphor described in Patent Document 5, such as Ca1.97Si5N5:Eu0.03, contains no cadmium, but the phosphor has such a low brightness that its emission intensity is insufficient.
Patent Document 12 discloses a silicon nitride phosphor activated with at least one rare-earth element indispensably containing Ce, a typical example of the phosphor being expressed by Ca2(Si, Al)5N8:Ce. According to the description, the phosphor can produce more colors than a known phosphor expressed by Sr2Si5N8:Ce3+. Patent Document 13 discloses an oxynitride phosphor containing Eu2+ ions as the luminescent center, the phosphor is typified by Sr2Al2Si3O2N6:Eu, and emits a warm color light or red light.
The phosphor composed of an inorganic compound having the same crystal structure as a CaAlSiN3 crystal, which is known as a heat-resistant material, as the host crystal, and an optically active element, preferably Eu2+, as the emission center emits an orange or red light having particularly high brightness. Accordingly, Patent Document 14 (hereinafter referred to as JP 2006-8721) describes a white light emitting device composed of the phosphor, the device providing high emission efficiency, and exhibiting good color rendering with an abundant red component.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-363554    Patent Document 2: U.S. Pat. No. 6,682,663    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-206481    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-322474    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2003-321675    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2003-277746    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2004-10786    Patent Document 8: Japanese Patent No. 2900928    Patent Document 9: Japanese Patent No. 2927279    Patent Document 10: Japanese Patent No. 3364229    Patent Document 11: Japanese Unexamined Patent Application Publication No. 10-163535    Patent Document 12: Japanese Unexamined Patent Application Publication No. 2004-244560    Patent Document 13: Japanese Unexamined Patent Application Publication No. 2005-48105    Patent Document 14: Japanese Unexamined Patent Application Publication No. 2006-8721    Nonpatent Document 1: H. A. Hoppe et al., “Journal of Physics and Chemistry of Solids” 2000, vol. 61, pages 2001-2006    Nonpatent Document 2: “On new rare-earth doped M-Si—Al—O—N materials” written by J. W. H. van Krevel, TU Eindhoven 2000, ISBN, 90-386-2711-4
The phosphor disclosed in JP 2006-8721, which is composed of an inorganic compound having the same crystal structure as the CaAlSiN3 crystal as the host crystal, is an excellent phosphor having a center emission wavelength of 653 nm and achieving high luminous efficiency.
In general, when a phosphor is used for a lighting system or display, the phosphor is required to have high luminous efficiency and emit light having an intended emission wavelength. This is because, when a phosphor is used for a lighting system, whether the color rendering or luminous flux is prioritized over the other depends on the use conditions. For example, when the luminescent center of a phosphor shifts to a highly visible green side, color rendering tends to deteriorate but the luminous flux increases. Therefore, a phosphor having an intended emission wavelength advantageously enhances the flexibility in designing a lighting system. When the phosphor is used for a display, the range of color reproducibility is adjustable according to the intended use, which enhances the flexibility in the display design.
JP 2006-8721 discloses a method for obtaining a phosphor having a shorter wavelength of the luminescent center, wherein Ca is partially substituted by Sr.